Generally speaking, a digital pen is a pen that is capable of writing on paper in ink, in the same manner as an ordinary pen, and in addition is capable of digitizing and then transferring or internally storing any marks made by such pen. Most of such digital pens utilize a miniature camera for detecting preprinted patterns made on the paper. By using paper with appropriately preprinted patterns and the proper decoding software, such a digital pen generally can identify the individual piece of paper, either uniquely or as a member of a particular set, on which marks are made and can distinguish where on the paper each mark is made. As a result, the digital pen can be used as a computer-input device. One example of a digital pen is the Hewlett-Packard Digital Pen, which stores pen strokes internally and then, upon being inserted into a cradle, downloads the digitized marks into a computer.
One way of marking paper with position-location information is described in U.S. Pat. No. 6,548,768 (the '768 Patent), which is incorporated by reference herein as though set forth herein in full.